When an oil well is first drilled and completed, the fluids (such as crude oil) may be under natural pressure that is sufficient to produce on its own. In other words, the oil rises to the surface without any assistance. In many oil wells, and particularly those in fields that are established and aging, natural pressure has declined to the point where the oil must be artificially lifted to the surface. Subsurface pumps are located in the well below the level of the oil. A string of sucker rods extends from the pump up to the surface to a pump jack device, beam pump unit or other devices. A prime mover, such as a gasoline or diesel engine, an electric motor or a gas engine, on the surface causes the pump jack to rock back and forth, thereby moving the string of sucker rods up and down inside of the well tubing.
The string of sucker rods operates the subsurface pump. A typical pump has a plunger that is reciprocated inside of a barrel by the sucker rods. The barrel has a standing one-way valve, while the plunger has a traveling one-way valve, or in some pumps the plunger has a standing one-way valve, while the barrel has a traveling one-way valve. Reciprocation charges a chamber between the valves with fluid and then lifts the fluid up the tubing towards the surface.
One problem encountered in downhole pumps is that the chamber between the valves fails to fill completely with liquid. Instead, the chamber contains undissolved gas, air, or vacuum, which are collectively referred to herein as gas.
Such failure to completely fill the chamber is attributed to various causes. In a gas lock situation or a gas interference situation, the formation produces gas in addition to liquid. The gas is at the top of the chamber, while the liquid is at the bottom, creating a liquid-to-gas interface. If this interface is relatively high in the chamber, gas interference results. In gas interference, the plunger (on the downstroke) descends in the chamber and hits the liquid-to-gas interface. The change in resistances causes a mechanical shock or jarring. Such a shock damages the pump, the sucker rods and the tubing. In addition, a loss of pumping efficiency results.
If the liquid-to-gas interface is relatively low in the chamber, a gas lock results, wherein insufficient pressure is built up inside of the chamber on the downstroke to open the plunger valve. The plunger is thus not charged with fluid and the pump is unable to lift anything. A gas locked pump, and its associated sucker rods and tubing, may experience damage from the plunger hitting the interface.
I am a co-inventor of U.S. Pat. No. 6,273,690, which addresses the problem of gas in the compression chamber by allowing the gas to bleed off from the chamber. The pump has worked very well.
In some instances, however, the gas remains in solution with the liquid in the compression chamber. Thus, any attempts to bleed off the gas are frustrated by the lack of separation between the gas and liquid. Consequently, the gas either interferes with, or else if present in sufficient quantities, locks the pump.
In the prior art, there are several types of gas separators used in conjunction with sucker rod downhole pumps. One type of prior art separator uses a dip tube located at the bottom of the pump. Surrounding the dip tube is a mud anchor, with a bull plug at the bottom. The mud anchor forms a chamber around the dip tube. The mud anchor has perforations, wherein the fluid enters the chamber through the perforations and travels down where it then enters the dip tube. The distance between the mud anchor perforations and the entry to the dip tube is referred to as the quiet zone, which is typically 1.5-2 times the pump volume. The fluid temporarily resides in the quiet zone on the pump downstroke, allowing gas to bubble out and escape through the mud anchor perforations.
Another type of prior art separator utilizes a stationary rotor. Fluid is forced into the angled rotor vanes to rotate the fluid, wherein gas is separated from the fluid. The reciprocating action of the pump moves the fluid through the rotor.